In the past, there has been much effort in developing systems for measuring the resistivity .rho. (resistance per unit area; inverse of conductivity .sigma.) of materials, usually liquids or solids, which are encapsulated, contained, or otherwise isolated by a dielectric (insulating) material in order to determine and analyze other medium characteristics. One reason, among others, is the development of new radar absorbing material (RAM). RAM may comprise a selectable amount of conductive materials encapsulated within insulating materials for permitting the varying of resistivity across the RAM.
Another reason for needing such a system is that federal and commercial product performance standards are becoming increasingly demanding and require an effective, low cost, non-invasive, and nondestructive way to measure characteristics of products which are contained or encapsulated. As examples, it may be necessary to detect the contamination level of industrial fluids, including water, liquid lubricants, and solvents, or it may be necessary to determine the amount of one substance within another, such as the quantity of fat, salt, or water, in food.
By measuring the resistivity of a material, the contents and other characteristics of the material can be determined because many of the contents have well defined resistivity levels. Well known systems for measurement of resistivity typically involve the use of a Wheatstone bridge or a Wien bridge. However, both of the foregoing techniques require direct contact of the bridge with the material under test. This predicament is unacceptable when evaluating a material which is encapsulated, contained, or otherwise isolated by a dielectric material. Furthermore, direct contact methods for measuring resistivity are generally not permissible when analyzing the contents of food, sterile substances, or other materials which are susceptible to undesirable contamination upon contact.
There are some situations in which the resistivity of limited size materials could be determined and analyzed with radio frequency (RF) transmission tests. The resistivity is measured by transmitting an RF electromagnetic signal to the material and receiving a reflected electromagnetic signal. The analysis is based on the fact that the transmission and reflection coefficients equal one, according to conservation laws, and the following relationships: EQU R=(.eta.-1)/(.eta.+1) (1) ##EQU1## where R=reflection coefficient, .eta.=normalized input impedance, .mu..sub.r =relative permeability, and e.sub.r =relative permitivity. However, practical test setups using this method are limited to laboratory type conditions and require expensive, cumbersome equipment in order to generate results which are generally plagued with errors and exhibit questionable accuracy at best. Furthermore, this method is very time consuming, is mathematically intensive, and requires a high level of skill to practice it.
U.S. Pat. No. 5,210,500 to Pingel et al. describes a system for contactless measurement of the electrical impedance and resistivity of a material using capacitive coupling and a circular coupling device having three electrodes. Although the system is not devoid of all merit, the system is complex and mathematically intensive in that it requires measurement and analysis of three different frequencies, phase, and stray capacitance. Moreover, the system has limited applicability.